1. Field of the Invention
This invention pertains generally to fluid treatment systems and more particularly to an apparatus for sensing at least one fluid parameter at at least two different locations in said system.
2. Prior Art
As a result of the ever increasing cost of energy there has been an increased demand in the air refrigeration industry, as in other industries, for systems designed to reduce energy consumption. For example, "Economizer Cycles", designed to shut down the refrigeration apparatus and provide free cooling to the conditioned space whenever possible are now common hardware in most air refrigeration systems.
More recently, air refrigeration systems have been designed to cool the outside air, as opposed to the return air, whenever the enthalpy of the former is lower of that of the latter. Essential to these systems are sensors disposed in appropriate locations throughout the system. Typically, the signals from these sensors are fed to a control center which then positions the system dampers to minimize energy consumption. Typical means for sensing air enthalpy in such systems include sensing the wet bulb temperature of the air or sensing two other air parameters, such as dry bulb temperature and relative humidity from which enthalpy is derivable. Exemplary enthalpy controlled air refrigeration systems are disclosed in U.S. Pat. No. 3,913,344 issued to Holloway, et al and U.S. Pat. No. 3,979,922 issued to Shavit.
One of the major drawbacks of any air refrigeration system which compares an air parameter at two spaced locations in the system by comparing signals generated by separate sensors disposed at these locations is that the validity of the comparison is dependent upon accurate calibration of the sensors. An invalid comparison could in turn result in improper system control with the result that energy is wasted.